Re: [PATCH net v2] xsk: fix NULL pointer dereference in __xsk_rcv()

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

On Fri, Jul 24, 2026 at 11:42:46PM -0400, Cen Zhang (Microsoft) wrote:
> In the __xsk_rcv() multi-buffer path, xsk_buff_alloc() is called in a
> loop without checking its return value. xsk_buff_can_alloc() only
> counts fill queue entries without validating their addresses, so it
> can succeed while xsk_buff_alloc() rejects all remaining entries and
> returns NULL.
> 
>   Oops: general protection fault, probably for non-canonical address
>    0xdffffc0000000000
>   KASAN: null-ptr-deref in range
>    [0x0000000000000000-0x0000000000000007]
>   RIP: 0010:__xsk_rcv+0x426/0xc20 (net/xdp/xsk.c:350)
>   Call Trace:
>    xsk_generic_rcv+0x26d/0x5f0
>    xdp_do_generic_redirect+0x3c5/0xcf0
>    do_xdp_generic+0x92f/0xe70
>    __netif_receive_skb_core.constprop.0+0xf7e/0x2b30
> 
> Fix this with a two-stage transaction. First allocate and stage all
> buffers required for the packet, recycling all staged buffers with
> xsk_buff_free() if any allocation fails. Only after this stage
> succeeds, copy the data, reserve the RX descriptors, and release the
> buffers in an error-free loop.
> 
> Fixes: 804627751b42 ("xsk: add support for AF_XDP multi-buffer on Rx path")
> Reported-by: AutonomousCodeSecurity@microsoft.com
> Signed-off-by: Cen Zhang (Microsoft) <blbllhy@gmail.com>
> ---
> v2:
>  - Allocate all packet buffers before reserving RX descriptors.
>  - Recycle partially allocated buffers instead of only cancelling the
>   RX producer reservations.
> Link: https://lore.kernel.org/netdev/20260724164719.99563-1-blbllhy@gmail.com
> 
>  net/xdp/xsk.c | 29 ++++++++++++++++++++++++++---
>  1 file changed, 26 insertions(+), 3 deletions(-)
> 
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index f906d51b6699..383fc2b1de48 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -298,9 +298,11 @@ static int __xsk_rcv(struct xdp_sock *xs, struct xdp_buff *xdp, u32 len)
>  	u32 frame_size = __xsk_pool_get_rx_frame_size(xs->pool);
>  	void *copy_from = xsk_copy_xdp_start(xdp), *copy_to;
>  	u32 from_len, meta_len, rem, num_desc;
> -	struct xdp_buff_xsk *xskb;
> +	struct xdp_buff_xsk *xskb, *tmp;
>  	struct xdp_buff *xsk_xdp;
> +	LIST_HEAD(xsk_buffs);
>  	skb_frag_t *frag;
> +	u32 i;
>  
>  	from_len = xdp->data_end - copy_from;
>  	meta_len = xdp->data - copy_from;
> @@ -343,23 +345,44 @@ static int __xsk_rcv(struct xdp_sock *xs, struct xdp_buff *xdp, u32 len)
>  		frag =  &sinfo->frags[0];
>  	}
>  
> +	for (i = 0; i < num_desc; i++) {
> +		xsk_xdp = xsk_buff_alloc(xs->pool);
> +		if (!xsk_xdp)
> +			goto err_alloc;
> +
> +		xskb = container_of(xsk_xdp, struct xdp_buff_xsk, xdp);
> +		if (unlikely(!list_empty(&xskb->list_node)))
> +			goto err_alloc;
> +		list_add_tail(&xskb->list_node, &xsk_buffs);

could we use existing xsk_buff_add_frag() ?
then I presume xsk_buff_free() would understand list and walk through
xdp_buff's and free it ?

I believe we could reuse pool's xskb_list instead of fabricating the
on-stack variant here.

> +	}
> +
>  	do {
>  		u32 to_len = frame_size + meta_len;
>  		u32 copied;
>  
> -		xsk_xdp = xsk_buff_alloc(xs->pool);
> +		xskb = list_first_entry(&xsk_buffs, struct xdp_buff_xsk,
> +					list_node);
> +		list_del_init(&xskb->list_node);
> +		xsk_xdp = &xskb->xdp;
>  		copy_to = xsk_xdp->data - meta_len;
>  
>  		copied = xsk_copy_xdp(copy_to, &copy_from, to_len, &from_len, &frag, rem);
>  		rem -= copied;
>  
> -		xskb = container_of(xsk_xdp, struct xdp_buff_xsk, xdp);
>  		__xsk_rcv_zc_safe(xs, xskb, copied - meta_len,
>  				  rem ? XDP_PKT_CONTD : 0);
>  		meta_len = 0;
>  	} while (rem);
>  
>  	return 0;
> +
> +err_alloc:
> +	list_for_each_entry_safe(xskb, tmp, &xsk_buffs, list_node) {
> +		list_del_init(&xskb->list_node);
> +		xsk_buff_free(&xskb->xdp);
> +	}
> +	xs->rx_dropped++;
> +	return -ENOMEM;
>  }
>  
>  static bool xsk_tx_writeable(struct xdp_sock *xs)
> -- 
> 2.53.0
>